ProtoXEP: Explicit Mentions

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This proposal defines a way to explicitly mention one or more users in messages, providing functionality commonly found in legacy messengers. The main benefits this proposal has over current regex-based implicit mentions are:

• Ability to mention various groups of users at once
• Ability to NOT mention a user when saying their name, resulting in fewer false positives
• Client-side ability to fine-tune mention-related notifications
• Server-side ability to configure mention-related permissions and filter malicious mentions accordingly

This ProtoXEP was originally written by pep back in 2023, who has graciously allowed me to build upon their work and submit it. The original version relied on a change proposed to XEP-0372 found in #1281 which hasn't been approved yet. The current version of this proposal removed the dependency on XEP-0372, but I'd like to add it back if the change is approved; I've specifically designed this proposal to make the revision as easy as possible to make, when the time comes.

I'm new to writing standards as well as to XMPP, but I've tried to copy the way other XEPs work and incorporate feedback from various people as best I can. In particular, I'm least confident that permissions have been specified correctly, so feedback on that would be especially appreciated.

[CLAassistant]

All committers have signed the CLA.

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Discussion in xsf@ the other day reminded me to actually implement this. For the curious, here's a Slixmpp plugin implementing most of the proposal other than MUC permissions which I wrote for a friend to use with their bot.